The last several decades have brought significant changes to broadcast radio technology. First came amplitude modulation (AM) analog radio, then, in turn, frequency modulation (FM) analog radio, FM-AM and FM multiplex stereo analog radio, FM discrete stereo analog radio, FM multiplex quadraphonic analog radio and, finally, AM discrete stereo analog radio. Some of these radio technologies proved longer-lasting than others. However, the latest in high-tech broadcast radio, Satellite Digital Audio Radio Service, or SDARS, is capable of providing a new level of service to the subscribing public.
SDARS promises to overcome several perceived limitations of prior broadcast forms. All such prior forms are “terrestrial,” meaning that their broadcast signals originate from Earth-bound transmitters. As a result, they have a relatively short range, perhaps a few hundred miles for stations on the AM and FM bands. Therefore, mobile broadcast recipients are often challenged with constant channel surfing as settled-upon stations slowly fade and new ones slowly come into range. Even within range, radio signals may be attenuated or distorted by natural or man-made obstacles, such as mountains or buildings. Radio signals may even wax or wane in power or fidelity depending upon the time of day or the weather. This only serves to compound the motorists' frustration.
Additionally, broadcast radio is largely locally originated. This constrains the potential audience that can listen to a particular station and thus the money advertisers are willing to pay for programming and on-air talent. While the trend is decidedly toward large networks of commonly-owned radio stations with centralized programming and higher-paid talent, time and regulatory change are required to complete the consolidation.
Finally, the Federal Communications Commission (FCC) defined the broadcast radio spectrum decades ago, long before digital transmission and even digital fidelity were realizable. The result is that the bandwidth allocated to an FM radio station is not adequate for hi-fidelity music, and the bandwidth allocated to an AM radio station is barely adequate for voice.
SDARS promises to change all of this. A user who has a SDARS receiver in his vehicle can tune into any one of a hundred or more nationwide stations with the promise of compact disc (CD) quality digital sound. By virtue of satellite redundancy, transmission from overhead and transcontinental coverage substantial provides immunity to service interruption. Satellite technology appears to be doing for radio what cable and satellite technologies have done for television.
While SDARS uses satellites for broad-area coverage, SDARS calls for terrestrial repeaters capable of transmitting relatively strong signals to augment the satellites by filling-in data when and where satellite signals may be too weak. Satellite signals are particularly prone to being weakened in urban areas due to the buildings, bridges and tunnels.
To promote competition in SDARS, the U.S. Government has divided the 25 MHz S-band allocated to SDARS into two equal 12.5 MHZ subbands and licensed those subbands to two separate companies: Sirius (www.sirius.com) and XM (www.xmradio.com). Each company operates its own independent service, including its own constellation of satellites and its own network of terrestrial repeaters, located mostly of course in urban areas.
Unfortunately, because terrestrial repeater signals tend to be stronger than satellite signals and because the two SDARS services occupy proximate subbands, interference can degrade the two services. A particular concern arises when a terrestrial repeater of one service introduces noise into the satellite signals of the other service. The noise plays havoc with the way SDARS receivers interpret the signals they are trying to receive. It is especially troubling that the urban areas where interference is most likely to occur is where most SDARS subscribers are located.
Accordingly, what is needed in the art is a way to handle interference occurring in SDARS signals and a SDARS receiver that is more immune to signal interference.